The present invention relates to a carbon monoxide sensor for detecting the a concentration of carbon monoxide in fuel gas having a high content of hydrogen used, for example, in a fuel cell.
Recently, a fuel cell using a solid high polymer proton conductive film is intensively developed for a home appliance and an automobile. The fuel cell uses hydrogen gas as fuel gas for operation, and hence requires a reformer for producing hydrogen gas by reforming liquid fuel such as methanol or utility gas. However, a fuel gas mainly composed of hydrogen produced by the reformer contains very little carbon monoxide of about scores of ppm. Carbon monoxide which is adsorbed in a platinum catalyst composing electrodes of the fuel cell (this phenomenon is called poisoning) lowers an electromotive force of the fuel cell. Consequently, it is necessary to monitor concentration of carbon monoxide in the fuel gas and to control the fuel cell accordingly.
A carbon monoxide sensor for detecting carbon monoxide in fuel gas for a fuel cell has been disclosed, for example, in Japanese Laid-open Patent No. 8-327590.
A sectional view of a schematic structure of this carbon monoxide sensor is shown in FIG. 13. An electrolyte film 110 is composed of a high polymer having proton conductivity. On both surfaces of the film, electrodes 112, 114 having a carbon cloth containing kneaded carbon powder and a platinum catalyst carried thereon are bonded by performing hot pressing. Mesh metal plates 116, 118 are disposed on surfaces of the electrodes 112, 114, on which the electrolyte film 110 is not disposed, respectively. The electrolyte film 110, electrodes 112, 114, and metal plates 116, 118 are held by flanges 120a, 122a provided inside of metal cylindrical holders 120, 122. An o-ring 126 for sealing gas is disposed at an electrolyte film 110 side at an end of the holder 122.
On an outer circumference of the holders 120, 122, threaded portions 120b, 122b are formed. The holders 120, 122 are fixed by being driven into threaded portions 124a, 124b formed inside of an insulating member 124 made of polytetrafluoroethylene, such as TEFLON (a trademark of Du Pont).
At one end of the holder 120, one end of a gas influent passage 128 is connected, from which object gas (fuel gas in this case) is introduced into the carbon monoxide sensor. One end of the holder 122 is not connected to the gas influent passage 128 and opens to an atmosphere.
Another end of the gas influent passage 128 is connected to a branch port 140a, provided at a part of a fuel gas passage 140, to allow gas to be introduced into the fuel cell.
Detecting terminals 120T, 122T are provided at the holders 120, 122, and an electric circuit 130 is connected to these terminals. The electric circuit 130 is composed of a voltmeter 132, and a resistor 134, for adjusting the load current, connected in parallel with the voltmeter. The detecting terminals 120T and 122T are connected to negative and positive electrodes, respectively.
An operation of the carbon monoxide sensor will be explained. An object gas (fuel gas) containing much hydrogen gas reaches the electrode 112 through the gas influent passage 128. The electrode 114 always contacts oxygen gas in the atmosphere. Therefore, on a surface of the electrolyte film 110 contacting the electrodes 112,114, hydrogen gas and oxygen gas react similarly to that as in the fuel cell to generate an electromotive force between the electrodes 112 and 114. The resistor 134 connected between the electrodes 112 and 114 creates a specified load current flow, and the voltmeter 132 detects a voltage between the electrodes 112 and 114.
In this situation, if carbon monoxide is mixed in the object gas, the carbon monoxide is adsorbed into the platinum catalyst at the electrode 112 and poisons the electrode 112. As a result, hydrogen gas and oxygen gas are prevented from reacting and voltage between the electrodes 112 and 114 is lowered. Since concentration of carbon monoxide relates to a degree of poisoning, by measuring voltage between the electrodes 112 and 114, a concentration of carbon monoxide in the object gas can be detected.
In this carbon monoxide sensor, to the extremely thin electrolyte film 110 made of high polymer sealing the object gas from the atmosphere, a differential pressure between object gas and atmosphere (usually several atmospheres) is always applied. In such circumstance, an abnormally high pressure, or an unexpected large pressure due to vibration or the like, is applied, especially at a portion designated by the circle in FIG. 13. This pressure may breaks the electrolyte film 110, and object gas with a high content of hydrogen gas may leak out to the atmosphere. To avoid such breakage, provided is a structure having a safety valve for releasing object gas in case of abnormal pressure. In any case, leakage of object gas into the atmosphere cannot be avoided.
Japanese Patent Laid-open No.11-219716 discloses another carbon monoxide sensor for detecting carbon monoxide in fuel gas containing much hydrogen gas supplied in a fuel cell.
A perspective exploded view of a schematic structure of this carbon monoxide sensor is shown in FIG. 14. An electrolyte film 50 is an electron exchange film composed of a high polymer having proton conductivity, for example, NAFION (a trademark of Du Pont). On opposite sides of the film, an anode 42 and cathode 44 containing catalyst particles are disposed. Conductive diffusion portions 43 and 45 made of carbon paper contact the anode 42 and cathode 44, respectively. The conductive diffusion portion 43 contacts a housing 54 having an object gas inlet 59, an anode flow channel 46 in which an object gas flows, and an object gas outlet 51. The cathode 44 is exposed to ambient air through an opening 52 of the housing 54. A metal current collector plate 49 in which a plurality of holes are formed contacts the conductive diffusion portion 45 and transfers current to a terminal 47. The terminal 47 projects outwardly from the housing 54 through a slot 55.
An operation of the carbon monoxide sensor will be explained. An object gas (fuel gas) having much hydrogen gas reaches the anode flow channel 46 through the object gas inlet 59. From here, the gas passes through the conductive diffusion portion 43 and is exhausted from the outlet 51. The cathode 44 always contacts oxygen gas in an atmosphere. Therefore, on a surface of the electrolyte film 50 contacting the anode 42 and cathode 44, hydrogen gas and oxygen gas chemically react, similarly to how they react in the fuel cell, so as to generate electricity by using hydrogen gas and oxygen gas, and thereby generates an electromotive force between the anode 42 and cathode 44. Current and voltage at this time are detected by a current detecting device and a voltage detecting device (not shown) connected between the terminal 47 and housing 54.
In this situation, if carbon monoxide is mixed in the object gas, the carbon monoxide is adsorbed into catalyst particles in the anode 42, and hence poisons the anode 42. As a result, hydrogen gas and oxygen gas are prevented from reacting, and the electromotive force between the anode 42 and cathode 44 is lowered. Since concentration of carbon monoxide varies depending on a degree of poisoning, by measuring a current change or voltage change due to drop of an electromotive force, a concentration of carbon monoxide in the object gas can be detected.
A resultant measurement of an output characteristic of the carbon monoxide sensor is indicated with a broken line in FIG. 10. It takes several minutes from introducing object gasp containing 50 ppm of carbon monoxide, into the carbon monoxide sensor for sensor output or electromotive force to change. During this period, voltage or current from the carbon monoxide sensor does not change, and therefore voltage change or current change cannot be measured. Thus, a concentration of carbon monoxide cannot be detected.
The invention presents a carbon monoxide sensor that includes:
(i) a proton conductive electrolyte film;
(ii) a detector composed of electrodes having catalysts disposed at opposite sides of the electrolyte film, and to be disposed in object gas containing hydrogen gas;
(iii) a power source having positive and negative terminals; and
(iv) a current detecting unit for detecting a current generated in the detector depending on the concentration of carbon monoxide in the object gas.
In this carbon monoxide sensor, the object gas does not leak to an exterior atmosphere.
A carbon monoxide sensor having a short response time is also presented which includes:
(i) a proton conductive electrolyte film;
(ii) an electrode having catalysts disposed at opposite sides of the proton conductive electrolyte film;
(iii) a positive electrode current collector plate including a gas passage having a gas inlet and outlet, and disposed so that the gas passage may contact one surface of the electrode;
(iv) a negative electrode current collector plate having a plurality of holes disposed so as to contact another surface of the electrode;
(v) a direct-current power source having positive and negative terminals connected to the positive electrode current collector plate, and negative electrode current collector plate, respectively; and
(vi) A current detecting unit for detecting a current that varies depending on a concentration of carbon monoxide in an object gas, that includes hydrogen, flowing in the gas passage.